1. Field of the Invention
This invention relates to a motor drive circuit.
2. Description of the Related Art
When driving a motor with an H-bridge circuit, it is necessary to prevent MOSFETs from being destroyed by a rise in a voltage caused at an occurrence of a kickback.
Japanese Patent Application Publication No. 2005-269885 discloses a method to suppress the rise in the voltage at the occurrence of the kickback using a Zener diode, for example. However, the method requires increasing a size of the Zener diode as a size of the motor increases, resulting in a higher cost of the motor drive circuit.
On the other hand, Japanese Patent Application Publication No. 2007-259657 discloses a motor drive circuit that prevents the destruction of the MOSFET at a low cost by suppressing the rise in the voltage at the occurrence of the kickback.
The motor drive circuit disclosed in Japanese Patent Application Publication No. 2007-259657 is specifically described hereafter, referring to FIG. 3. The motor drive circuit is formed to include P-channel type MOSFETs 11 and 12, N-channel type MOSFETs 13 and 14, power supply lines 21 and 22, a ground line 23, diodes 24 and 25, capacitors 26 and 27, electric current sources 31 and 32, NPN type transistors 41-48, PNP type transistors 51 and 52, a control circuit 60 and a connector 70. Each of the MOSFETs 11-14 is provided with each of parasitic diodes 11d-14d, respectively. A gate of each of the P-channel type MOSFETs 11 and 12 is connected to the power supply line 22 through each of resistors 33 and 34, respectively. The power supply lines 21 and 22 are shunted from a power supply 80 that generates a voltage VA, and are connected with a positive voltage side of the power supply 80 through the connector 70. The ground line 23 is connected with a negative voltage side of the power supply 80 through the connector 70.
When the MOSFETs 11 and 14 are turned on and the MOSFETs 12 and 13 are turned off in the structure described above, an electric current flows from the power supply line 21 to the ground line 23 through the MOSFET 11, a motor coil 10 and the MOSFET 14 to rotate the motor. Then, when the MOSFET 11 and the MOSFET 14 are turned off at a certain timing, energy accumulated in the motor coil 10 works to keep the electric current flowing. For that reason, the electric current flows through the parasitic diode 13d, the motor coil 10 and the parasitic diode 12d. In other words, a kickback is caused. The electric current caused by the kickback can be not recovered to the power supply 80 for regeneration because of the diode 24, and ends up in flowing into the capacitor 26. As a result, a voltage Vm on the power supply line 21 is raised. Then, the MOSFETs 11 and 12 are turned on when the voltage Vm on the power supply line 21 rises enough so that a voltage between a gate and a source of each of the MOSFETs 11 and 12 exceeds a threshold voltage. Therefore, the electric current outputted from the motor coil 10 returns to the motor coil 10 after flowing through the MOSFETs 12 and 11. That is, the energy accumulated in the motor coil 10 is dissipated in a loop composed of the motor coil 10 and the MOSFETs 12 and 11. Also, the rise in the voltage Vm on the power supply line 21 is suppressed to prevent the MOSFETs 11 and 12 from being destroyed by means of turning the MOSFETs 11 and 12 on.
As described above, with the motor drive circuit disclosed in the Japanese Patent Application Publication No. 2007-259657, the rise in the voltage Vm on the power supply line 21 at the occurrence of the kickback can be suppressed to prevent the MOSFETs 11 and 12 from being destroyed without using a Zener diode that is used in the method disclosed in Japanese Patent Application Publication No. 2005-269885.
With the motor drive circuit disclosed in Japanese Patent Application Publication No. 2007-259657, however, there are cases in which the loop composed of the motor coil 10 and the MOSFETs 12 and 11 is formed even when the voltage VA of the power supply 80 is reduced to less than a predetermined voltage (including a case where the power supply 80 is turned off). When the power supply 80 is turned off while the motor is rotating, for example, the motor keeps rotating for a while due to inertia. Then, varying magnetic flux in the motor coil 10 induces a voltage. In this case, the voltage at the source of each of the MOSFETs 11 and 12 rises since the voltage Vm on the power supply line 21 rises in response to the induced voltage. On the other hand, a voltage at the gate of each of the MOSFETs 11 and 12 is practically equal to zero when the power supply 80 is turned off in accordance with a voltage Vcc on the power supply line 22. Therefore, the MOSFETs 11 and 12 are turned on despite the power supply 80 is turned off. In this case, the motor coil 10 causes a braking force that is opposite in direction to the rotation of the motor.
This invention is directed to solve the problem addressed above, and intends to prevent the braking force from being caused when the power supply voltage is reduced to less than the predetermined voltage by avoiding the formation of the loop while suppressing the rise in the voltage at the occurrence of the kickback at a low cost.